This invention relates to a substrate bias circuit for setting a potential of a substrate in a semiconductor integrated circuit, etc., and more particularly, to a substrate bias circuit for use in a semiconductor integrated circuit driven by a single power supply.
In a MOSFET integrated circuit driven by a single power supply, a certain voltage is applied to a substrate terminal for the purpose of controlling a threshold voltage of a MOS transistor. Namely, in the case of an N-channel transistor formed on a P-type substrate, a negative potential is applied from the source terminal to the substrate terminal. On the other hand, in the case of a P-channel transistor on an N-channel substrate, a positive potential is applied from the source terminal to the substrate terminal. The circuit for generating such a bias voltage is called a substrate bias circuit.
A conventional substrate bias circuit is comprised of an oscillation circuit oscillating at a predetermined frequency, a control signal generation circuit supplied with this oscillation output, to generate a control signal, and a charge pump circuit for pulling down a substrate potential in response to this control signal.
However, in such a conventional substrate bias circuit, the efficiency of the charge pump circuit is poor in the state where the power supply voltage is low is poor. As a result, a sufficient bias effect cannot be provided.